One type of integrated circuit (IC) memory is called eraseable programmable read-only memory (EPROM). This type of memory permits the user to program the memory and, if desired, erase and reprogram memory. One type of EPROM is an N-channel metal oxide semiconductor field effect transistor (MOSFET), one example of which is shown in FIGS. 1 and 2, comprising a floating gate transistor 10 with two gates 12, 14 made of polysilicon. Polysilicon is commonly deposited by the thermal decomposition (pyrolysis) of silane (SiH4) or disilane (Si2H6) at about 520–700° C. using low-pressure chemical vapor deposition (LPCVD) techniques. The deposition at lower temperatures, such as 520° C., results in the deposited polysilicon being amorphous; the amorphous polysilicon can be recrystallized by subsequent annealing procedures, such as at about 900–1000° C. Gate 12 is a select or control gate while gate 14 is a floating gate. Transistor 10 comprises a substrate 16 including a source 18 and a drain 20 separated from one another by a channel 22. Floating gate 14 is separated from channel 22 by a first insulation layer 24, also called the gate oxide 24, while control gate 12 is separated from floating gate 14 by a second insulation layer 26.
FIG. 1 illustrates a conventional method for programming transistor 10. The arrow in FIG. 1 illustrates channel hot electron injection into floating gate 14 from channel 22 adjacent to drain 20 and through first insulation layer 24 causing a negative charge to be trapped within floating gate 14. The existence of the negative charge within floating gate 14 causes the threshold voltage of the programming transistor to the higher than prior to programming. The programmed transistor retains its programmed state even when the power supply is turned off; it has been estimated that the programmed device will retain its programmed state for as long as 100 years. FIG. 2 illustrates the erasing of transistor 10. The arrow in FIG. 2 suggests Fowler-Nordheim (FN) electron tunneling current through first insulation layer 24 and into source 18 (or along channel 22). Transistor 10 is read by applying a voltage between high and low threshold values to control gate 12. If transistor 10 is programmed, that is storing a 0, it will not conduct; if transistor 10 is not programmed, that is storing a 1, it will freely conduct.